Pebble in the Sky was Isaac Asimov's first ever published Science Fiction novel and it is one of the books in the
Empire series which is the precursor to the
Foundation series. It includes
The Currents of Space and
The Stars, Like Dust. This series was written very early on in his writing
career and it sometimes shows. Also, over time it was proved that many of the
Science Fiction elements in this series were actually incorrect. Things like
radiation's effect on
humans and such.
As such, the series is not in print anymore. However, if you frequent used book stores you can usually get a set in a couple of years.